The invention relates to a digital security image, to be arranged on a carrier, particularly a document, such as bonds or other documents the authenticity or origin of which is of importance.
In practice it is for instance common to provide a document that may or may not be provided with an image, with a security element against unauthorized copying. Such a security element may for instance be a security image, the security image being incorporated in the image and not or hardly being perceptible to the human eye.
To that end documents were in the past provided with a security characteristic that disappeared from the copy when copying, as a result of which it was detectable whether a document was authentic or a copy with which fraud might have been committed.
Additionally documents were in the past provided with a security characteristic that remained detectably present on the copy when copying. As a result it could be established whether a copy originated from an original or was counterfeit. An example of this is given in NL-A-9201701, in which as a security characteristic a first image invisible to the human eye has been incorporated in second image. Said second image is subsequently applied on a document. When making a photo copy of the document either the second image becomes visible to the eye, or the photocopying machine refuses to print the document.
Even if such characteristics were simultaneously arranged on one document, the various types of images were up until now arranged in different images or at different locations on a document. The drawback of this is that as a result the security characteristics take up a lot of space on a document to be secured. In addition detection of the various characteristics takes place separately. Because detecting the various characteristics often is highly arithmetic, in case it is a characteristic that can be made visible by means of specific image processing techniques, it is in many cases not or hardly possible to carry out the verification real-time, for instance at a check-out in a store. Additionally, verification of large numbers of documents is time-consuming.
An additional problem occurring with the present security elements is that it cannot be indicated whether a non-authentic document is a copy of an original or a complete forgery, particularly not when one security characteristic has been arranged.